This application is based on Japanese Patent Application Nos. 2001-030187 and 2001-030189 filed Feb. 6, 2001 and 2002-27473 filed Feb. 4, 2002, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which create color images of high quality on a print medium by ejecting different kinds of ink from a plurality of print heads. More particularly, the invention relates to improvements on an ink jet printing apparatus and an ink jet printing method which form a printed image by performing a single print scan (one pass) or multiple print scans (multiple passes) over one print area.
The present invention is applicable to all devices that use such print media as paper, cloth, leather, non-woven cloth, OHP sheet and even metal. Applicable devices include, for example, office equipment such as printers, copying machines and facsimiles and industrial manufacturing devices.
2. Description of the Related Art
Because of its advantages such as low noise, low running cost and the ease with which the apparatus can be reduced in size and with which color printing can be introduced, the ink jet printing method has found wide uses in printers, copying machines and facsimiles.
Generally, a color ink jet printing apparatus prints color images using four colors of ink, i.e., three color inksxe2x80x94cyan, magenta and yellowxe2x80x94plus a black ink.
In the conventional ink jet printing apparatus, to obtain color images with high color saturation without a spread of ink requires the use of dedicated paper that has an ink absorbing layer. This increases the printing cost. To solve this problem, a printing apparatus has been developed in recent years which is made applicable to inexpensive xe2x80x9cplain paperxe2x80x9d used in large volumes in printers and copying machines by improving the inks used. However, the print quality on the xe2x80x9cplain paperxe2x80x9d has not yet attained a satisfactory level. The most notable factors for this are color deviations due to differences in the order of ink ejection and in the ejection time. These cause problems such as density and color deviations or deviations in the feed direction (sub-scan direction) and transverse direction (main scan direction in which the print heads are scanned) of the print medium.
Further, in the ink jet printing apparatus there are growing user needs for the printer ability to print at high speed.
When printing is done with emphasis placed on high speed, the multipass printing, which is intended to realize high quality printing, is not performed but a so-called one-pass printing is effective which prints one line in one pass. The number of passes for printing means the number of times that the carriages is scanned to complete one line.
The reason for choosing the one-pass printing is that, because the print head has a fixed number of nozzles, the greater number of passes results in a smaller feed distance that the paper is fed at one time and the fewer passes result in a larger feed. For example, if a 2-pass printing currently adopted can be changed to a one-pass printing, this can simply increase the print speed two-fold. That is, the smaller the number of passes in the multipass printing, the smaller the number of carriage scans (number of scans over a predetermined area of the print medium) and the greater the distance that the paper is fed at one time. As a result, the time it takes for one sheet to be printed decreases.
By performing a bidirectional printing in which printing is done in both the forward and backward passes of the print heads, idle scanning can be eliminated, further increasing the print speed.
For example, FIGS. 1A to 1C show comparison between different printing methods using the same print head and the same number of scans. Here, the width of the print head is defined to be one line.
FIG. 1A shows a printed state after three scans have been performed by a 2-pass bidirectional printing. The example shown here represents a case where an image is completed by two passes with 50% duty printing performed in each pass. FIG. 1A shows a print area under consideration divided into four areas each corresponding to one-half of the print width of the print head H. In a first scan in the forward direction, the top area in FIG. 1A is printed with a 50% duty by using the lower half of the print head H. In a second scan in the backward direction, the top area of FIG. 1A is printed by using the upper half of the print head H and at the same time the second area from the top is printed with a 50% duty by the lower half of the print head H. This second scan completes the printing on the top area of FIG. 1A. In a third scan in the forward direction, the second area from the top is printed by the upper half of the print head H, completing the printing on this area. At the same time, the lower half of the print head H prints on the third area from the top with a 50% duty. If the area corresponding to the print width of the print head H is taken as one line, the execution of three scans completes one line and leaves one line half-finished, as described above. Although the example of FIG. 1A shows a bidirectional printing, if the printing is done in one of the forward and backward directions, there is an idle scan, which do not execute printing, between the successive scans. The completed area therefore is 0.5 line.
In the 1-pass unidirectional printing shown in FIG. 1B, three scans produce two lines of print-completed area. Here, there is one scan that does not execute printing (which in FIG. 1B corresponds to a backward idle scan following the forward scan). In a 1-pass bidirectional printing shown in FIG. 1C, three scans produce three lines of print-completed area and this method carries out the most efficient printing operation. For high speed printing, this is an effective method.
When a conventional ink jet printing apparatus forms an image of multiple colors by 1-pass bidirectional printing, however, a problem arises that the density deviations and color deviations due to differences in the order of ink ejection and in the ejection time degrade the image quality as described below.
Difference in the Order of Ejection
When print heads of different color inks are arranged in the transverse direction (main scan direction), because the order of ejecting the inks during the forward pass differs from that during the return pass, a hue difference may occur between the printing in the forward pass and the printing in the backward pass.
For example, when a blue, a secondary color, is formed, there are two different processes, as shown in FIGS. 2A and 2B. In the first case magenta is printed first producing a blue with a strong hue of magenta. In the second case cyan is printed first to produce a blue with a strong hue of cyan. This may be explained as follows. The hue depends on the characteristics of inks and paper, and the ink printed first is adsorbed first by paper fibers and the subsequently printed ink sinks under the first printed ink. The coloring component of the first printed ink exists mostly at or near the surface of the print medium and thus the intensity of the color of that ink, i.e., the density of that ink, is relatively high.
In a state where such a phenomenon occurs, if the 1-pass bidirectional printing as shown in FIG. 1C is performed, bands of different hues are printed alternately in the forward pass (during which the print head moves from left to right) and backward pass, as shown in FIG. 3. A first print head H1 and a second print head H2 eject magenta and cyan inks, respectively, with the magenta ink printed first in the forward pass and the cyan ink printed first in the backward pass. The difference in hue caused by the alternating order of ink ejection make the printed area look like a pattern of two bands of different hues alternating every scan.
Variations in Print Time
When print heads of different color inks are arranged in the vertical direction (sub-scan direction), the order of printing the different inks on each print area can be made the same. However, in the 1-pass bidirectional printing, because one print area is printed by alternately performing the forward and backward scans on a plurality of different print heads, the time interval between the previous printing operation and the subsequent printing operation differs from one location to another in each print area. This results in ink density deviations. In addition, the successive print areas adjoin each other in such a way that their time interval deviations become maximum in the sub-scan direction.
One such image formation is shown in FIG. 4.
Here, a cyan print head (second print head H2) and a magenta print head (first print head H1) are used to form a blue. In the print head shown in FIG. 4, the upper half is a print head H2 for cyan and the lower half is a print head H1 for magenta, with print heads H1 and H2 aligned in the sub-scan direction. In a first print area, the magenta ink is printed first in the forward pass. Next, in this first print area the cyan ink is printed in the backward pass. During this backward pass, a second print area is printed with the magenta ink at the same time that the cyan ink is ejected on the first print area.
Next, in the second print area the cyan is printed in the forward pass, during which time the magenta is printed in a third print area. In the first print area, the print time difference between cyan and magenta is large on the left side in the figure with respect to the main scan direction shown by an arrow but gradually decreases toward the right. In the second print area, this is reversed, with the print time difference between cyan and magenta increasing toward the right and gradually decreasing toward the left.
Hence, in the end portions of the print areas, printed areas with large print time differences and printed areas with small print time differences are alternated in the sub-scan direction. The printed area with a larger print time difference has a stronger hue of the first-printed ink. As the print time difference decreases, the hue shifted to the first-printed ink is alleviated. This is explained as follows. When the second ink is ejected after the first ejected ink has soaked into the print medium and well fixed there, the coloring component of the first ejected ink adheres well to the surface or close to the surface of the print medium, so that the intensity of the hue of the first ejected ink, i.e., the density of the first ejected ink, is high.
As the print time difference becomes small, the first landed ink is less fixed when the next ink lands on the medium. In that case, the coloring component of the first ink has not yet adhered well to the surface or close to the surface of the print medium, allowing the color component of the subsequently ejected ink to adhere to the surface or close to it. As a result, the hue is not heavily shifted to the first ejected ink but is neutral with both of the inks almost uniformly mixed, although the hue of the first ink may be slightly stronger. Because of this phenomenon, an image formed has the printed areas of different hues alternated close together, making the hue deviations appear as color deviations.
The present invention has been accomplished to overcome the above-mentioned problems. It is therefore an object of this invention to provide an ink jet printing apparatus and an ink jet printing method which can realize both a high speed printing and a high quality printing without color deviations when each of the print areas is printed in a plurality of scans by a plurality of print heads ejecting different kinds of ink.
Another object of this invention is to prevent color deviations caused by deviations in the ink ejection time interval between a plurality of print heads in each of the print areas when each print area is printed in a plurality of scans by a plurality of print heads ejecting different kinds of ink to form an image.
To solve the problems described above, the present invention has the following configurations.
A first aspect of this invention provides an ink jet printing apparatus comprising: printing means having a plurality of print heads, each having a plurality of ink ejection nozzles formed therein, the print heads ejecting different kinds of ink to print on a print medium; and print head control means for controlling for each print head independently widths and positions of active nozzle groups in the print heads, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; wherein the print head control means controls the widths and positions of the active nozzle groups according to a set printing condition.
A second aspect of this invention provides an ink jet printing apparatus comprising: printing means having a plurality of print heads, each having a plurality of ink ejection nozzles formed therein, the print heads ejecting different kinds of ink according to print data associated with the print heads to print on a print medium; print head control means for controlling for each print head independently widths and positions of active nozzle groups in the print heads, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; and boundary detection means for detecting boundaries between a plurality of print data associated with the print heads; wherein the print head control means controls the widths and positions of the active nozzle groups according to a detection result produced by the boundary detection means.
A third aspect of this invention provides an ink jet printing apparatus comprising: printing means having a plurality of print heads, each having a plurality of ink ejection nozzles formed therein, the print heads ejecting different kinds of ink to print on a print medium; active head portion control means for controlling for each print head independently widths and positions of active head portions in the print heads, the active head portions representing those of the plurality of nozzles in the print heads which are used for printing; and print medium information retrieving means for retrieving information on the print medium; wherein the active head portion control means controls the widths and positions of the active head portions according to the information retrieved by the print medium information retrieving means.
A fourth aspect of this invention provides an ink jet printing method for printing on a print medium by ejecting different kinds of ink from a plurality of print heads, each having a plurality of ink ejection nozzles, the ink jet printing method comprising: a printing condition setting step; a print head control step for controlling for each print head independently widths and positions of active nozzle groups in the print heads according to a set printing condition, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; and a printing step for printing by the plurality of print heads according to the widths and positions of active nozzle groups set by the print head control step.
A fifth aspect of this invention provides an ink jet printing method for printing on a print medium by ejecting different kinds of ink from a plurality of print heads, each having a plurality of ink ejection nozzles, according to print data associated with the print heads, the ink jet printing method comprising: a boundary detection step for detecting boundaries between the plurality of print data associated with the print heads; a print head control step for controlling for each print head independently widths and positions of active nozzle groups in the print heads according to a detection result produced by the boundary detection step, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; and a printing step for printing by the plurality of print heads according to the widths and positions of active nozzle groups set by the print head control step.
A sixth aspect of this invention provides an ink jet printing method for printing on a print medium by ejecting different kinds of ink from a plurality of print heads, each having a plurality of ink ejection nozzles, the ink jet printing method comprising: a print medium information retrieving step for retrieving information on the print medium; a print head control step for controlling for each print head independently widths and positions of active nozzle groups in the print heads according to the retrieved information on the print medium, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; and a printing step for printing by the plurality of print heads according to the widths and positions of active nozzle groups set by the print head control step.
With the above configuration, the optimum widths of the active nozzle groups for the selected print mode can be used for printing. For example, in the high speed printing mode the largest nozzle group widths within the applicable range are specified for printing. In the high quality print mode, limitations are imposed on the use of the active nozzles in printing to prevent image impairments such as color deviations. This arrangement can provide a printing apparatus that can realize both a high speed printing and a high quality printing.
A seventh aspect of this invention provides an ink jet printing apparatus comprising: printing means having a plurality of print heads, each having a plurality of ink ejection nozzles formed therein, the print heads ejecting different kinds of ink to print on a print medium; and print head control means for controlling for each print head independently widths and positions of active nozzle groups in the print heads, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; wherein the print head control means controls the positions of the active nozzle groups according to a set printing condition.
An eighth aspect of this invention provides an ink jet printing method for printing on a print medium by ejecting different kinds of ink from a plurality of print heads arranged in printing means, each print heads having a plurality of ink ejection nozzles, the ink jet printing method comprising: print head control step for controlling for each print head independently widths and positions of active nozzle groups in the print heads, the active nozzle groups representing those of the plurality of nozzles in the print heads which are used for printing; and wherein the print head control step controls the positions of the active nozzle groups according to a set printing condition.
With the above configuration, the optimum widths of the active nozzle groups for the selected print mode can be used for printing. For example, in the high speed printing mode the largest nozzle group widths within the applicable range are specified for printing. In the high quality print mode, positions of the active nozzle groups that will not cause image impairments, such as color deviations, are specified for printing. This arrangement can provide a printing apparatus that can realize both a high speed printing and a high quality printing.
A ninth aspect of this invention provides an ink jet printing apparatus comprising: printing means having a plurality of print heads, each having a plurality of ink ejection nozzles formed therein, the print heads ejecting different kinds of ink to print on a print medium; and active head portion control means for controlling for each print head independently widths and positions of active head portions in the print heads and print timings of the print heads, the active head portions representing those of the plurality of nozzles in the print heads which are used for printing; wherein the active head portion control means controls a print time interval between the plurality of the print heads in each print area according to a set printing condition.
A tenth aspect of this invention provides an ink jet printing method for printing on a print medium by ejecting different kinds of ink from a plurality of print heads, each having a plurality of ink ejection nozzles, the ink jet printing method comprising: a active head portion control step for controlling for each print head independently widths and positions of active head portions in the print heads and print timings of the print heads, the active head portions representing those of the plurality of nozzles in the print heads which are used for printing; wherein the active head portion control step controls a print time interval between the plurality of the print heads in each print area according to a set printing condition.
With this invention, because the print time interval between the print heads in each print area is controlled according to the set printing condition, it is possible to prevent print time interval deviations in each print area.
For example, in this invention, the widths and positions of the active nozzle groups in the print heads can be limited according to the selected printing condition. Therefore, when a high speed print mode or a high quality print mode is selected as the printing condition, the number of active nozzles and their positions are specified according to the printing condition. This allows either a high speed printing or a high quality printing to be selectively performed as required. Further, since the ink ejection time interval between the print heads in each print area can be controlled, it is possible during the high quality printing to make the ink ejection time interval in each print area constant, thus preventing color deviations in each print area.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.